The present invention relates to a semiconductor device provided with a circuit board that includes an insulated substrate on both sides of which are joined a first metal plate and a second metal plate, a semiconductor element joined to the first metal plate, and a heat radiating device thermally joined to the second metal plate via a junction member.
In general, a circuit board of a power module, which is a semiconductor device, includes an insulated substrate formed of aluminum nitride. First and second metal plates are respectively joined to both sides of the insulated substrate. A semiconductor element is thermally joined to the first metal plate, and a heat radiating device is thermally joined to the second metal plate. The heat radiating device radiates heat generated by the semiconductor element. The heat radiating device in such a power module is required to maintain heat radiating performance for an extended period of time. However, depending on the use conditions of the power module, thermal stress is generated in the heat radiating device by the difference in coefficient of linear expansion between the insulated substrate and the heat radiating device. As a result, cracks may occur in the insulated substrate or at junction portions between the insulated substrate and the heat radiating device, and a warp may be generated at the junction section of the heat radiating device joined to the insulated substrate.
Japanese Laid-Open Patent Publication No. 2004-153075discloses a power module that solves such a problem. The power module of the publication includes an insulated substrate, a radiator, and a heat sink. A heating element (semiconductor element) is mounted on one side of the insulated substrate and the radiator is soldered to the other side of the insulated substrate. The heat sink is fastened to the radiator with screws. The radiator includes a pair of plate-like radiator main bodies formed of a material having a high thermal conductivity such as aluminum and copper and a low thermal expansion material such as an invar alloy. The pair of radiator main bodies sandwich the low thermal expansion material. However, in the power module of the above publication, since the heat sink is joined to the radiator by screws, the thermal conductivity between the heat sink and the radiator is not high. As a result, heat generated in the heating element is not efficiently radiated.
Japanese Laid-Open Patent Publication No. 2006-294699discloses a heat radiating device of a semiconductor device that efficiently radiates heat generated in a heating element (semiconductor element). The heat radiating device of the above publication includes an insulated substrate and a heat sink. A heating element is mounted on one side of the insulated substrate, and a metal layer and the heat sink are joined to the other side of the insulated substrate. A stress relaxation member is located between the insulated substrate and the heat sink. The stress relaxation member is formed of a material having a high thermal conductivity and has stress absorption spaces. The stress relaxation member has metallic joints joined to the insulated substrate and the heat sink. Thus, the thermal conductivity between the insulated substrate and the heat sink is high. As a result the heat radiating performance for radiating heat generated in the heating element is improved.
In recent years, it has been desired to further improve the thermal conductivity from the heating element to the heat radiating device while reducing thermal stress caused by heat generated in the heating element (semiconductor element).